1. Field of the Invention
This invention relates to light-sensitive positive photoresist compositions, and more particularly to radiation-sensitive compositions comprised of an o-naphthoquinone diazide and a phenolic resin or resole having enhanced application among others, to the fabrication of semiconductor photomasks and integrated circuit devices.
2. Description of the Prior Art
A positive photoresist in use today for microlithography applications is comprised of an alkali soluble polymer such as a novolak (e.g. phenol-, cresolformaldehyde, etc.) or resoles and a photoactive component (PAC) such as a diazoquinone, preferably an o-naphthoquinone diazide, as a sensitizer for the resist. Such sensitizers and resists are described among others in U.S. Pat. Nos. 3,046,118, 3,046,121, 3,106,445, 3,201,239 and 3,666,473, which are incorporated herein by reference. When the resist is image-wise exposed to radiation, the exposed areas dissolve in alkaline solution (developer). In this way, image-wise exposure of the photoresist layer is utilized to produce relief patterns of the resist on a substrate for purposes of, for example, making exposure masks or resist patterns such as are employed in the fabrication of integrated circuit devices.
The major portion of the positive type resists are composed of a novolak resin or resole, a diazoquinone component (PAC) and casting solvent(s). The universal application of novolak type polymers to diazoquinone resist systems is based on the aqueous alkaline solubility of these resins. This property is essential to the exploitation of diazo photochemistry, i.e. conversion of non-alkaline soluble compounds to an alkali soluble carboxylic acid by UV radiation. For a given type of diazo PAC there exists an optimum resin to PAC ratio, which provides maximum lithographic performance in a well-defined process. This optimum ratio can seldom be achieved in high-volume resist production facilities because of batch-to-batch variations in resin alkali solubility and/or PAC casting solvent solubility. In general, the main factor dictating PAC to resin ratio in a given resist lot is resin alkali solubility. Each new resist lot is forced to behave like the last lot by adjusting PAC loading, up or down, to compensate for alkali solubility of the particular resin batch being used. PAC solubility places constraints on these adjustments. When resin alkali solubility is relatively high, then PAC loading required for good performance may exceed PAC solubility limits. This problem occurs repeatedly in various and different positive resist lots of this type, and can make several resin lots unuseable for resist applications. In addition, steps often are required to be taken to improve PAC solubility, since even resins with moderate alkali solubility require PAC loading which approaches solubility limits. One approach to improvisng PAC solubility normally employed in the technology involves purity reduction which has a negative impact on resist performance, particularly in E-beam lithography. The overall problem of resin to PAC mismatch is compounded by the inability of providing resins with lower, more desirable alkali solubilities. Table I below shows alkali solubility data for seven lots of novolak resins, with the resins listed in chronological date order of their manufacture. Rate value show how quickly a resin-only film (no PAC) of each lot is solubilized by a dilute alkaline developer. Rate values are relative indicators of PAC loading requirements to make a functional resist.
TABLE I ______________________________________ CRESOL-FORMALDEHYDE NOVOLAK RESIN ALKALI SOLUBILITY DISSOLUTION LOT RATE (.ANG./MIN) ______________________________________ A 3300 B 1600 C 3200 D 3200 E 9800 F 4800 G 8000 H 5700 ______________________________________
The data in the Table indicates a significant trend toward higher alkali solubility with the latter resin lots. Accompanying this trend is the need to load still greater amounts of PAC, which of course has practical limits. The most successful (highest overall resist manufacturerability and performance) is Lot B, which also has the lowest alkali solubility. By contrast, Lots E and G possess the highest alkali solubility but are impractical for resist applications because of PAC solubility limits. Efforts to reproduce Lot B performance in new resin lots have failed because in the complexity of feed-stocks and synthesizing variables associated with this resin. Thus it is difficult to insure a continuing consistent supply of useable resin. Accordingly it would therefore be desirable to devise a technique for reducing novalak alkali solubility without adversely affecting other important resin properties, such as film integrity, gel content, UV transparency, sensitivity, etc.